Existing cable television systems (1) scramble programs, (2) transmit the scrambled programs to customers, and (3) provide the customers with xe2x80x9cset-top boxesxe2x80x9d for unscrambling the programs. Many such television systems operate in the analog signal domain.
However, digital signal transmission is coming into widespread use. Digital signal transmission possesses characteristics which are different from analog signal transmission. The invention presents novel modes of transmission and encryption which are especially suited to the unique capabilities of digital signal transmission.
In one form of the invention, multiple decryption keys are used to decrypt encrypted material which is transmitted to customers of a restricted-access television system.